It is usually rather easy to copy and duplicate electrically presented information, which makes it more difficult to control that copyrights are observed. According to international agreements, copyright to a certain image belongs to the producer, (author) of the image. If the producer saves the image in the form of an electric file in a computer, which is connected to a wide information network, such as, the Internet, it is possible that another user of the network copies the image file and uses it in other contexts without paying compensation to the producer, who is entitled to it on the basis of the copyright. If the producer wants to present a legal claim for compensation, he must be able to prove that the image used elsewhere is a copy of the image produced by the person claiming compensation. It would be very important for the producer that even after the transfer and possible processing measures it would be possible to find some information indicative of the origin of the image from the file transferred in electric form. However, this information must be so hidden that a dishonest user cannot remove it or even perceive its existence.
Methods used to indicate ownership of image files can be divided into two main categories. In watermark methods, a graphic element is added to the image, which element can be isolated afterwards and shown as a graphic evidence to prove that it has been added to the image on purpose, and that it is not a random combination of parts of the image. In checksum methods, a checksum or a logical combination is calculated by a secret method from the bits and/or bytes used to present the image, and this combination is added to the image file as part of it. The origin of the image is detected by calculating the same logical combination afterwards and comparing it to the original checksum contained by the file. Because only the real owner of the image knows the correct calculation method for the checksum, the probability of getting the same sum with a method chosen at random is small.
All the above mentioned methods have the following common objectives:
the visual quality of the image must not suffer from the addition of the authentication code to the image, PA1 only the producer and possibly an independent, reliable testing institution can check the authentication code hidden in an image, PA1 the authentication method must allow the use of many kinds of authentication codes or "signatures", PA1 the authentication code must remain in the image with a sufficient reliability, even if the image is packed with the JPEG or MPEG algorithm, for example, and PA1 the authentication code must endure attempts to pirate the images or to cover the real origin of the image.
In watermark methods, the graphic element used as a authentication code is generally called a watermark or signature. One known, watermark-type method has been presented in the publication R. G. van Schyndel, A. Z. Tirkel, C. F.
Osborne. A Digital Watermark, Proceedings of ICIP-94, Volume II of III, pp. 86-90, IEEE, Austin, Tex., 1994. For other publications dealing with prior art methods for authentication coding of images, please see the reference list at the end of the descriptive part of this application. Generally in the prior art publications, the signature is converted to a virtually random form, the pixels of which are scattered on the image to be authentication coded. A disadvantage of these methods is the fact that the calculation method is very complicated.